WO 93/04940 has disclosed a dispensing device for simultaneously dispensing two fluid substances.
This dispensing device has a first reservoir, which is delimited by an inner side of a first cylindrical tube, and a second reservoir, which is delimited by the outer side of the first cylindrical tube and an inner side of a second cylindrical tube. The first and second tubes are arranged concentrically with respect to one another, the second tube surrounding the first cylindrical tube. On the side remote from the outlet side, the first reservoir is delimited by a continuous disk-like piston and the second reservoir by an annular piston.
In the known dispensing unit, the two reservoirs are filled from the underside, where the piston is located. Filling from the top is not possible, since both the inlet valve and the outlet valve of the first and second pumps do not allow the fluid substance to flow in the opposite direction. Therefore, the reservoir is filled at its still open underside, after which the corresponding piston is moved into the reservoir which has been filled before. One drawback of positioning the piston afterwards is that air is present between the fluid substance and the piston in the reservoir.
This air which is present in a reservoir means that the volume which is subsequently dispensed by the pump in one pump stroke is not always constant. This is undesirable in particular in the case of dispensing units which dispense two fluid substances in a defined volumetric ratio, since a slight difference in volume in the fluid substance dispensed can cause a considerable deviation in the intended volumetric ratio between the two fluid substances dispensed. The latter problem occurs in particular if the difference in volume between the first fluid substance dispensed during a pump stroke and the second fluid substance is considerable.
There are also known reservoir assemblies in which—after the filling via the open underside—a piston provided with a closable opening is put in place. Air can escape via this opening, after which the opening is closed off. Closing off the opening in the piston then requires additional operations to be carried out during the filling of the reservoir assembly.
In general, therefore, it is not easy to fill the known reservoirs of the abovementioned type and it is difficult to prevent air from being present between the piston and the fluid substance.